In recent years, an increase in the number of subscribers to mobile communication services has created a need to expand capacity of a radio zone that each wireless base station covers. One of multiple access techniques responding to this need is Space-division Multiple Access (SDMA).
SDMA is a technique for a wireless base station to communicate with a plurality of mobile stations at the same time at the same frequency by dividing a space. The division of space is achieved by the wireless base station by generating the most suitable directivity pattern to each mobile station. The directivity pattern represents the direction and the strength of radio waves to be radiated or received.
The wireless base station comprises, as a mechanism for forming directivity patterns, a plurality of antennas and a Digital Signal Processor (DSP). When transmitting or receiving signals, the wireless base station forms directivity patterns as desired by weighting the amplitude and phase of each signal radiated or received via each antenna. Here, each value used for weighting is referred to as a weighting factor, and a group of weighting factors used for forming one directivity pattern is referred to as a weight vector.
The wireless base station, using an adaptive array method, follows a mobile station that issues a signal of which incoming direction is unknown so as to form the most suitable directivity pattern thereof. One of the principles achieving this operation is Minimum Mean Square Error (MMSE). The MMSE technique requires a reference signal. The reference signal is a signal serving as a target for a signal obtained by weighting each reception signal to be as close as possible. The wireless base station employing the MMSE technique determines a weight vector in a manner to minimize the difference between the reference signal and the signal obtained by weighting. Using the weight vector determined in this way, the directivity pattern will be of the most suitable one for receiving a signal transmitted from the intended mobile station.
On the other hand, in the case of another multiple access technique of Time-division Multiple Access/Time-division Duplex (TDMA/TDD), each slot includes a bit pattern, such as a preamble and a unique word, which are known to a receiving end, preceding a signal indicative of actual information.
Thus, when communicating using a combination of MMSE-based SDMA and TDMA/TDD, the wireless base station may use a preamble, a unique word, or the like as a reference signal.
To be more specific, the wireless base station 1) sets an intended weight vector as an initial value, 2) compares a reference signal such as a preamble, unique word, or the like with the actual signal obtained by weighting using the weight vector so as to obtain an error therebetween, and 3) adjusts the weight vector value in a manner to minimize the error. Repeating this operation for each symbol of the bit pattern, the weight vector value converges to one value with the passage of time, so that the signal indicative of the actual information is extracted by weighting using the weight vector converged. After receiving the known bit pattern such as the preamble or the unique word, the wireless base station identifies a symbol represented by the extracted signal and uses the signal corresponding to the symbol as the reference signal, assuming that the identified symbol is correct. When transmitting, the wireless base station transmits a signal using the weight vector calculated at the time of reception that has taken place immediately before.
In order to prevent interference possibly caused by SDMA and to maintain adequate communication quality during communication, the wireless base station judges whether each slot is in a suitable condition for space-division multiplexing, and prohibits space-division multiplexing in a time slot judged not suitable while permitting space-division multiplexing in a time slot judged suitable. There are two ways to make the suitability judgment.
One way is to make the judgment based on a correlation value between reception-response vectors of two mobile stations that are intended to be space-division multiplexed. The wireless base station calculates reception-response vectors of each of the two mobile stations and then a correlation value between the two reception-response vectors. Here, the response vector is information about the incoming direction of a signal from a mobile station, and the correlation value serves as an index showing how close the incoming directions of signals from two mobile stations are. In other words, a relatively large correlation value indicates that the two mobile stations are located in approximately the same direction, so that it is considered impossible to separate the signals using different directivity patterns. Thus, the wireless base station judges that space-division multiplexing is not suitable when the correlation value exceeds a predetermined threshold value.
The other the way is to make the judgment based on an electric field strength ratio between signals from two mobile stations that are intended to be space-division multiplexed. The wireless base station obtains electric field strengths of the signals from the two mobile stations using the reception-response vectors and calculates the electric field strength ratio between them.
A relatively large electric field strength ratio indicates that the electric field strength ratio exceeds the antenna gain ratio however suitable the directivity patterns may be formed, so that it is impossible to properly separate the two signals. Thus, the wireless base station judges that space-division multiplexing is not suitable when the electric field strength ratio exceeds a predetermined threshold value.
Yet, there may be a case where the wireless base station fails to calculate weight vectors of two mobile stations and to form appropriate directivity patterns although it has been judged in the conventional technique mentioned above that space-division multiplexing is suitable.
When the directivity patterns are not appropriately formed, there may be a case where a mobile station that newly requests allocation of communication channel can not start communication or where a mobile station already in communication is disconnected. This leads to the above-mentioned problem that quality of communication with each mobile station is not always ensured.